By maintaining a high Ones density, this approach permits the payload to be contained within three levels, along with clocking information.

Because of this encoding method, the observed highest frequency component is 772 kHz, half that of T1's 1.544 Mbits/s bit rate. The lower frequency associated with this three-level coding approach makes it possible to use a repeater every 6000 feet (maximum) that can recover a signal with up to 32 dB of degradation (if the full signal bit-rate of 1.544 Mbits/s had to be carried on this same wire, the resulting attenuation would make the service impractical).

LAN Applications
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Multi-level signaling has played an important role in advancing local-area network (LAN) traffic beyond its initial 2.94 Mbits/s bit rate and the use of large-diameter coaxial cable. Some engineers found they could use less expensive UTP cable by incorporating the MLT-3 three-level coding method. MLT-3 alternates in a sinusoidal pattern (..., 0, +1, 0, -1, 0, +1, ...) that repeats indefinitely, as indicated in this diagram.

Note that a logical One causes the progression to continue, while a logical Zero halts the progression, preventing a transition to the next level in the sequence.

MLT-3 is used in 100Base-TX (Fast Ethernet), which relies on one pair of wires for the transmit direction and one pair for the receive direction. These methods reduce the bandwidth of the transmitted signal to one-fourth of the original data-transmission rate—stretching the signal-handling capability of the media.

GbE Extends the Principles
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GbE extends the principles of multi-level coding through the use of five-level (quinary) pulse-amplitude modulation encoding. In PAM5 encoding, each transmitted symbol represents one of five different levels (-2, -1, 0, +1, +2), as shown here.

Note that four levels are used in PAM5 to represent two bits; the extra fifth level supports forward error-correction (FEC) coding (4-dimensional 8-state Trellis coding).

As the next diagram shows, the multi-level signals in GbE are passed in parallel across multiple wires, and are recombined at the receiver, with each wire pair operating in full-duplex mode.

Note that each wire pair can achieve a throughput of 250 Mbits/s using baseband signaling at 125 Mbaud—providing 1 Gbits/s at a spectral power density similar to that of 100Base-TX.

Significantly, this permits the use of FR4 backplanes and copper wire. Because GbE can use existing CAT-5 UTP and CAT-5 RJ-45 connectors, it's easy to upgrade existing LAN infrastructures cost-effectively to have GbE capability.

Better Signal-to-Noise Ratios
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The reduction in transmission bandwidth brought about by multi-level signaling lessens frequency-dependent signal attenuation. That results in lower channel distortion and a significant improvement in signal-to-noise ratio (SNR) as the number of levels increases, as shown in this graph comparing SNR attenuation vs. the number of levels in a multi-level signal.

On the other hand, trans-hybrid losses, dielectric losses, and cabling-return-loss losses combine with near-end and far-end crosstalk resulting from the use of unshielded, adjacent wire pairs. Consequently, high-frequency effects would seem to erode the SNR in UTP and FR4 materials.

In practice, however, the use of hybrid transformers, echo cancellers, adaptive equalization, and FEC coding can overcome much of this SNR degradation. In fact, the use of multi-level signaling methods actually results in an overall increase in the SNR for applications such as GbE and backplane electronics. That's true because the gain in SNR resulting from a decreased operating frequency more than offsets the loss in SNR resulting from loss and attenuation.

The Test Challenges
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For all its benefits, multi-level signaling imposes significant test challenges that can threaten delays in the volume delivery of GbE and backplane devices.

For one thing, you'll need to measure analog amplitude, voltage, and distortion at a finer resolution consistent with the multi-level signals. At the same time, you'll need to resolve these complex signals to their high-speed digital data streams to verify device function.

With their sophisticated capabilities, new multi-level devices present test engineers with a potentially lengthy list of test procedures. To establish an industry-wide test baseline, the IEEE has defined several tests in its IEEE Std. 802.3ab-1999 publication, which is a supplement to the IEEE Std.802.3.

In describing these tests, this standard refers to the IEEE 802.3 CSMA/CD LAN model, which defines the various reference layers. This diagram shows the 1000Base-T PHY and ISO OSI reference model.

Note that these IEEE recommendations emphasize the electrical characteristics of the physical-layer (PHY) device since compatibility between separate implementations must be proven at this layer.

To provide a more effective test foundation, the IEEE standards also specify that each IEEE-compliant GbE device must provide four transmitter test modes, as well as a Gigabit Media-Independent Interface (GMII) management register for controlling these modes.